Effects of Interlayer Properties on the Performance of Unbonded Concrete Overlays

Unbonded concrete overlays (UBOL) consist of a new Portland cement concrete (PCC) layer placed on an existing PCC pavement. The new concrete layer is separated from the existing pavement by an interlayer system, allowing these overlays to be placed on distressed PCC pavements. The interlayer system usually consists of a thin open graded or dense graded hot mix asphalt (HMA) layer or a non-woven geotextile fabric. In an effort to develop a mechanistic-empirical design procedure for UBOLs, the effects of the interlayer properties on the performance of the overlay must first be established. There are many variables to consider when selecting an interlayer for UBOLs such as, drainability, frictional restraint, prevention of reflective cracking, and stability of a mixture to prevent erosion or consolidation/ rutting. Performance data from in-service pavements at the Minnesota Road Research Facility (MnROAD), Michigan, Missouri, and Pennsylvania, as well as data in the Long-Term Pavement Performance (LTPP) database is examined to establish the influence of interlayer characteristics on overlay performance. . Many of the distresses are at least partially caused by breakdown of the interlayer. Longitudinal cracking develops in the wheelpath due to erosion or consolidation of an asphalt interlayer, while transverse cracking can occur due to reflective cracking, reflective distress, or erosion along the transverse joint. The characteristics of the interlayer also play a role in the rate of the development of faulting. Based on this evaluation, factors that should be considered for optimizing the performance of the interlayer are defined.